Changing env vars "upstream"? (how do you change MPD_HOST?)

Question

on desktops, you usually have a process tree that goes something like (well, this is only one branch):
init───login───bash───startx───xinit─┬─.xinitrc───sessionmanager─┬─WM
we all know that env vars are passed on "down the tree". but this sometimes bothers me, for example when using mpd. it uses a variable MPD_HOST
so when i have a running desktop, and i want to use an other mpd instance I must change MPD_HOST.  I can only do this in the "local" environment.  the widgets in my WM, the interfaces that I use etc cannot be aware of the changed variable.
how do you guys handle this?
I have some ideas for solutions:
1) make MPD_HOST=mpdhost and use /etc/hosts to make it point to other things. but that won't help for MPD_PORT or non-mpd related variables.  not mentioning some apps might cache dns internally. and this requires root
2) hack my session manager so that i can somehow notify it (signals, let it listen to a socket, etc) and have it change an environment variable.  but i don't know if the changed variable will automatically "propagate downstream" (probably not).
any more ideas?
thanks.

Answer 1

hm yeah, it obviously won't work for running instances of programs - it just makes sure that changes in env (in the respective env-file) take effect for all new instances of $program.
You can eg use gdb to change the env of a running process (have a look at this script for reference http://www.nanonanonano.net/linux/chenv) , but most of the time that won't help you. Typically env vars are only evaluated at startup and then used to populate the appropriate structures. You'll probably have to implement the required functionality yourself in every single program.
For MPD a more or less simple workaround might be writing a small proxy that allows you to switch its destination on the fly - but i don't know whether MPD's protocol is stateful and whether clients will handle the transition gracefully.
Last edited by raf_kig (2010-02-12 14:55:02)